More and more, we hear and read about the importance of the microbiome: the ecological community of friendly and unfriendly microorganisms that share our body space. The human body contains as much as ten times more microbial organisms than human cells, and increasingly, changes to the microbiome are being associated with both good and bad alterations to our health.

Blood Type Connections

Most laypeople and physicians can be forgiven for wondering what blood type could possibly have to do with the microorganisms in the digestive system. Blood type, after all, is just an ornament on a red blood cell, right?

Simply a complication of transfusion.

Sadly, not much else about blood type is taught as part of the standard medical curriculum. However, readers of my books and followers of my diet theory already know that there is a much deeper and important role that blood type plays in conditioning our digestive tract; both in terms of the characteristics of the lining (mucosa) and the variations of the secretions (digestive juices) produced by the stomach and intestines. These blood type specific differences can themselves have an important effect in altering the microbial balance of the digestive tract. But there are even more elemental and direct ways that blood type can condition the microbiome.

The blood type antigens (the marker that distinguishes one ABO blood type from another) are widely distributed throughout the digestive tract, embedded in the mucus lining that insulates the gut tissue from the rather harsh internal environment. The most common blood group antigens are proteins that contain the sugar fucose. This the most basic building block, and is called the H antigen. Although we commonly think of blood type O and not possessing a blood group antigen, the name ‘O’ was chosen to signify the number zero. The rationale behind that label only relates to the fact that type O blood does not trigger an immunologic reaction when transfused into the other ABO types. Type O blood does in fact possess an antigen, the so-called ‘H’ antigen. However, since all the other ABO types also make H antigen, it is not immunologically reactive in them, and allows type O to function as the ‘universal donor.’ So, to tie this important relationship together, let’s put it in a nice infographic:

Blood Type Antigens as Bacterial Snack Food

There is good evidence that the ABO blood type antigens are a major influence on the specificity of the bacterial enzymes produced by the microbiome, serving to ‘condition’ the microbiome. Much like how putting sunflower seeds into your bird feeder will tend to encourage Cardinals to hang out in your backyard, the microbiome of a person who is blood type B will tend to have bacteria with a preference and enzyme capacity for degrading the B-antigen. Human feces contain enzymes produced by the bacteria of the microbiome that degrade (break down and metabolize) the A, B, and H blood type antigens lining the digestive tract and convert it to an energy source for their own use. The population of fecal bacteria that produce blood group-degrading enzymes is highly correlated with the ABO and secretor type of the host. In fact, blood group specificity is common among intestinal bacteria with almost 50% of strains tested showing some blood type A, B, or O specificity. To give you an idea of the magnitude of the blood type influence on intestinal microflora, it has been estimated that someone with blood type B will have up to 50,000 times more of some strains of friendly bacteria than either blood type A or O individuals. Thus, you might be surprised to discover than much of a healthy microbiome is the result of harboring bacteria that like to ‘eat right for their blood type.’

Blood Type Antibodies as Nightclub Bouncers

Although the average physician does not consider blood type antibodies beyond their role as a transfusion nuisance, common sense dictates that they serve a more basic role; natural selection obviously can not predict this type of need, despite its obviously life-saving benefits. Instead these antibodies work in combination with the antigens to create a molecular and immunologic ‘line in the sand,’ allowing microbial species with similarity to the antigen to avoid immune reaction, while targeting dissimilar strains for destruction. A large percentage of microorganism share antigenic similarity with the ABO blood type antigens, which in themselves are nothing unique or special to humans. Thus, if you are a critter who has developed a similar antigen to the blood type A antigen, you’ll probably do fairly well in the gut of blood type A humans. On the other hand you are probably going to have real difficulties should you find yourself inside the body of a type B, as they carry antibodies (extremely powerful ones, by the way) to type A and will not like you very much at all. Many studies have associated increases in the blood type antibodies during many common infections, and the ability of type O to manufacture both anti-A and anti-B antibodies may have given them a real advantage when it came to natural resistance to many pathogenic bacteria and viruses.

However, similarity between your blood type antigen and the antigens on a microbe are no guarantee of future harmony. Just as an uninvited guest can sometimes get past the nightclub bouncer by claiming that ‘I’m with the band,’ microbes with ill-intent can evade the basic defenses by developing the ability to mimic the blood type antigen of their potential host/victim. Just as in Star Trek, this ‘cloaking device’ gives them a degree of invisibility from the immune system. In one study, bacteria antigens that cross-reacted with blood group antigens were detected on cell walls of anaerobic bacteria from 30% of the cultures inoculated. Other microbial felons may have an advantage due to their possessing specialized ways to attach to the blood type antigens in the gut, which makes infection much easier. Many of these ‘molecular suction cups’ are adhesive-type lectins, a class of proteins also found in many foods.

A huge amount of research has been done correlating the differences in resistance to pathogenic microorganisms that exist between the different blood types. Historically, some of the most catastrophic epidemic and endemic diseases are ABO selective, and in many instances demonstrate ABO related differences in morbidity (sickness), mortality (death), or the level of the inflammatory response to the infection.

There are some very practical recommendations, which can help increase the diversity of microbes colonizing your gut:

Dr. Peter D'Adamo is a naturopathic physician, author, researcher-educator and software developer. He is considered a world expert on glycobiology, principally the ABO blood groups and the secretor (FUT2) polymorphisms. He is the author of the international best-seller, Eat Right 4 Your Type and the Blood Type Diet series of books, and he is currently a Distinguished Professor of Clinical Sciences at the University of Bridgeport, Connecticut.